Which $15 player runs Rockbox?
My main Rockbox'd unit is an iPod Video (5.5) with an iFlash Quad (up to 4x microSD cards) and a 2000mAh battery. Not the most expensive project, but not as cheap as the Clip+.
My guess is that the parent is talking about the Sansa c200 or something similar.
The best iPod Video is the Enhanced / 5.5 that has the 'search' item. An 80gb version of this also has more RAM, which I don't think is utilized at this time, but it might down the road. This may not be accurate now, but it was a few years ago.
The 80gb will come with the fat back. If you do the mod I did, you can use the slim back from the 30gb.